Artificial transmission line



y 8, 1951 N. E. BEVERLY 2,552,306

ARTIFICIAL TRANSMISSION LINE Filed Nov. 6, 1948 K A FIG- 3 NELSON E. BAIERLY INVENTOR.

FREQUENCY Patented May 8, 1951 UNITED STATES PATENT OFFICE SpragueElectric Company,

North Adams,

Mass., a corporation of Massachusetts Application November 6, 1948,Serial No. 58,650

Claims.

This invention relates to new and improved electrical circuits and moreparticularly refers to artificial transmission lines havingcharacteristics which are highly desirable, but heretofore have beenpractically unattainable. This application is a continuation-in-part ofmy copending U. S. patent application, Serial Number 673,726, filed June1, 1946, and entitled Artificial Transmission Line, patented October 17,1950, Number 2,526,321.

The classical picture of a transmission line is a network of an infinitenumber of sections, each consisting primarily of a series inductance(corresponding to the inductance per unit length of the wireconstituting the transmission line), a series resistance (correspondingto the resistance per unit length of the wire) and a parallelcapacitance (which is the capacity to ground or between adjacent wiresof a unit length of the wire). Such transmission lines are very useful,particularly in high frequencies, in transmitting or rejecting signals,in matching or transforming impedances, and in delaying and modifyingthe wave shape of signals.

It is well known in the art to make artificial transmission lines withso-called lumped characteristics, that is to say, made up of a finitenumber of inductances and capacities, and these artificial transmissionlines with lumped parameters have been quite successful. t highfrequencies they suffer from the defect, however, that the lumpedinductance has a high distributed capacity, and that the lumped capacityhas a high series inductance so that it is extremely difficult to obtainthe electrical equivalent of the natural transmission line. At the sametime the natural transmission line to have useful qualities is so longphysically that it is difficult, if not impossible, to make use of it inthe lower radio frequencies.

A further difficulty with the known artificial transmission lines isthat it is impossible to match the behavior of the natural line at thephysical start of the line. In the natural line there is shuntcapacitance at its inception. There is likewise series inductanceappearing in the natural line upon its inception. With the artificialline a choice must be made between starting with either a seriesinductance or a shunt capacitance, and this choice necessarily makes theresulting network diifer from the natural prototype.

It is an object of the present invention to overcome the foregoing andother disadvantages of the prior art. A further object is to produce newelectrical circuits having desirable characteristics. A still furtherobject is to produce an artificial transmissionline which combines theadvantages of prior art transmission lines without at the same timebeing subject to their disadvantages. A still further object is toproduce artificial transmission lines and networks of simple physicalstructure which have characteristics greatly desired but heretoforeunattainable. Additional objects will become apparent from aconsideration of the following description and claims.

These objects are attained in accordance with the present inventionwhich is concerned with an artificial transmission line comprising twolayers of electrically conducting materials separated by layers ofdielectric material and convolutely wound, one of the layers ofelectrically conducting material being connected on both sidesthroughout substantially its entire length to a terminal and the otherlayer of electrically conducting material being connected at its innerextremity only to an axial terminal which extends from both sides of thewinding. In a more restricted sense the invention is concerned with anartificial transmission line comprising a wide electrode foil and anarrow electrode foil separated by layers of dielectric material ofintermediate length and convolutely wound, the wide electrode foil beingconnected on both sides throughout substantially its entire length to aterminal and the narrow electrode foil being connected at its innerextremity to an axial terminal which extends from both sides of thewinding. In one of its preferred embodiments the invention is concernedwith an artificial transmission line comprising a wide electrode foiland a narrow electrode foil separated by layers of dielectric materialand convolutely wound upon an electrically conducting core which extendsbeyond both sides of the winding, the narrow electrode foil beingconnected at its inner extremity to said core and the wide electrodefoil being connected on both sides throughout substantially its entirelength to a metallic housing for said winding. In one of its limitedembodiments the invention is concerned with an artificial transmissionline comprising a wide electrode foil and a narrow electrode foilseparated by layers of dielectric material of intermediate width, thelongitudinal center lines of each of said foils registering with eachother and the resulting assembly being convolutely wound on a rigidmetal core which extends beyond the edges of said Winding, the innerturn of said narrow foil electrically engaging said core and theprojecting edges of the wide foil being embedded in solder and connectedto a cylindrical metal housing which encloses said convolutely woundassembly. The invention is also concerned with an electrical circuituseful as a transmission line and in bypassing undesired highfrequencies which circuit comprises the artificial lines described.above in which the housing and one end of the core are connected asinput terminals and the metal housing and the other end of the core areconnected as output terminals.

In the accompanying drawing:

Figure 1 shows a cross-section of an artificial transmission line of theinvention;

Figure 2 shows a schematic diagram of the line of Figure 1; and

Figure 3 ShOWs the insertion loss vs. frequency curve of an electricalcondenser (A) and of the transmission line of the invention (B).

Referring more specifically to Fig. 1, I represents an axialelectrically conducting core, which extends beyond both sides of theassembly. This core may be fabricated as a wire, rod, bar, plate orother metal configuration. Further, it may be employed as the mandrelfor the winding of the assembly. Silver-plated copper, tin-plated copperand aluminum are representative core materials. The extensions of H1 maybe provided with studs, clips or other devices to permit connectioninto, a circuit and also, in some cases, to facilitate physicalmounting.

The convolute winding consists of narrow electrode foil ll, wideelectrode foil I2 and intermediate width dielectric layers 13. Theextended edges of foil I2 are embedded in solder or similar conductingmaterial is and IS. The narrow foil ll contacts core In at its innerextremity !6. This contact may be soldered or welded, if desired.

Insulating bushings I! and I8 are provided on core IE), to insulate coreIi! from foil l2. These may be of rubber, plastic, ceramic or otherinsulating material. Metal housing l9 encloses the winding and iscrimped onto bushings H and I8 as shown at 29. Thus electrical contactis made to foil I2, through low resistance paths l4 and E5. At, the sametime, the assembly is physically sealed and protected from vibrationetc.

It is generally advisable to provide a metal clamp on housing [9, tosimplify mounting and electrical connection. A wide clamp is preferablefor lowest coupling impedance. Another useful mounting arrangement canbe'produced by welding or soldering a flange about the housing i9,preferably in the middle. This flange may be bolted, welded, soldered,threaded or otherwise connected into a metal wall and the device of theinvention used for feed-through filtering purposes. Forexample, thedevice could be used to remove unwanted high frequencies from a directcurrent power supply for an automobile radio.

Fig. 2 shows a schematic diagram in which narrow foil as is connected tocore 34. The ex tremities of the latter serve as input and outputterminals. Wide foil 3! is provided with low resistance paths 32 and 33throughout the winding and is connected to a housing and/or clamp, whichserves as an input and output terminal.

Fig. 3 shows the insertion loss frequency curves for a condenser and formy artificial transmssion line. In this instance, the condenser is ofthe so.- called non-inductive type, with one foil, extending beyond oneside of the winding. The artificial transmission line is of the typeshown in Fig. l and possesses the same electrical capacity tinmicrofarads) as the condenser, when measured at 60 cycles. The condenseror the, transmission line is placed as a, filter in shunt with a load,

so as to reduce or eliminate unwanted higher frequencies in the incomingsignal. In the case of the transmission line, one side of the line isconnected and fed through the core of the artificial transmission line;one end of the core is connected to the signal source and the other endof the core to the load. The metal housing is connected to the otherside of the line, which. is usually grounded. One terminal of thecondenser is connected to one side of the line and the other terminal tothe other side of the line, which, as aforesaid, is usually grounded.The effectiveness is measured by noting the ratio of voltage across theload at a particular frequency without and with the transmission line orcondenser as a shunt, the source current being maintained constant. Theratio is known as the insertion loss and is expressed in decibels. Thus,if a high value of insertion loss occurs at a certain frequency, thesignals of th-atfrequency passing through the load are considerablyreduced by the. bypassing action when the transmission line or condenseris in shunt with the load circuit. High insertion losses indicate thatthe impedance across the bypassing device is low. A high insertion lossis therefore desirable when the artificial iine or the condenser is tobe. employed as a filter circuit, for example, to remove high frequencySignals from a primarily direct current power source.

Referring now to Fig. 3 of the drawing, curve A represents the insertionloss for the condenser previously mentioned. As the frequency increases,the insertion loss increases until resonance occurs. At this point, theinsertion loss is very high and the filtering effectiveness isappreciable. As the frequency is further increased, however, theinsertion loss rapidly decreases and approaches a relatively low valueas a result of the appreciable inductance of the device. Curve B showsthe insertion loss of the artificial transmission line. At lowerfrequencies, it is substantially similar to the condenser. As theresonant frequency is approached, however, the insertion loss approachesa maximum and thereafter, at higher frequencies, the insertion loss ismaintained at or near this relatively high level. Thus its effectivenessasv a filter circuit element. for bypassing high frequencies ispronounced and far greater than that of the electrical condenser.

It is apparent that numerous modifications may be made of this inventionWithout. departing from the spiirt and scope thereof. Representativemodifications will be discussed, in the followin paragraphs.

The winding may be made with a. number of different, conducting layers.For example, paper may be metallized with zinc, aluminum, etc. to.provide a very thin but conducting layer on the dielectric spacer.Alternately carbon paper may be employed. Suitable electrode foilsinclude aluminum, copper, silver, lead, tin, etc. When the length ofthe. narrow foil is appreciable, it is generally advisable to employ afairly conductive material as the electrode material. For this purpose,copper, aluminum and silver are preferable.

The dielectric spacing material may be paper, a resin film, a resincoating on the electrode foil, a flexible ceramic coating on the foil,an oxide film on the. foil, or other material which will withstand theoperating voltages without breakdown. It is generally preferred toimpregnate the convolutely wound assembly with a dielectric oil, wax orresin. To this end, the impregnation may be carried out either before orafter placing the. wound assembly the housing,

The extended edges of the wide foil may be pressed together, soldered,welded, or otherwise interconnected to form a low resistance path to theterminal arrangement from each point the winding. The housing isgenerally metal so as to produce a low impedance terminating path to thechassis or other electrical circuit point to which it is to beconnected. A metal tube may be spun over the ends of the assemblyagainst the bushing or other insulation on the axial terminal elementsto the narrow foil. Alternately the winding may be wrapped in a flexiblefoil or provided with a sprayed metal coating. For some applications itis desirable to provide an insulating casing about the transmissionline. In such cases a resin may be cast or heat-and-pressure moldedabout the winding, totally enclosing the latter with exception of theaxial terminals to the narrow foil.

The axial terminal element which extends from both sides of the windingmay be insulated from the extended wide foil edges with a bushing of aplastic material, as aforementioned, such as a polyamide, polyethylene,rubber, polysiloxane, etc. In some instances, particularly for lowvoltage applications, the insulation may be simply cotton, silk or awire enamel. Where extremely high operation temperatures are to be met,ceramic or polytetrafiuoroethylene bushings may be employed.

The width and length of the electrode foils as well as the materials ofwhich they are made can be varied extensively to produce designsparticularly adapted to certain applications. Where relatively lowdistributed inductance is desired in the transmission line, the narrowfoil is shorter and wider than where a high distributed inductance isdesired.

My invention has been particularly described in connection withelectrostatic transmission lines. It is, however, quite possible toproduce electro lytic transmission lines. In such instances the narrowelectrode foil would be of a film forming metal such as titanium.aluminum, or tantalum and its surface would be provided with aninsulating oxide film of the metal. Instead if a dielectric spacer anelectrolyte-saturated spacer would be employed. The wide electrode ioilwould enerally be of unformed aluminum or other film forming materialsuitably interconnected if necessary, and terminated as previouslydescribed. The axial terminal rod or core to the narrow electrode foilwould be a film forming metal provided with the insulating oxide filmlayer thereon, to increase shunt resistance.

The use of cylindrical metal containers for the transmission lines ofthe invention is by no means critical, as the uncased wound assembliesthemselves, with a low impedance terminal or strip to the wide electrodefoil, may be employed. Further, the convolutely wound section may bepressed flat or square without any appreciable effect upon the highfrequency characteristics. The metal casing adds to the durability ofthe line, to the simplicity of mounting, and in some cases to thesimplicity of electric connection into the circuit.

The artificial transmission lines are usually incorporated in circuitsby connecting one end of the core as an input terminal and the other endof the core as an output terminal for one side oi the line. The terminalstrap or metal housing for the wide foil serves as both input and outputconnection for the other side 01' the line, which, in

most instances, is grounded to the chassis or container of the completecircuit assembly.

As many apparently widely diiferent embodiments of this invention may bemade without departing from the spirit and scope hereof, it is to beunderstood that the invention is not limited to the specific embodimentshereof, except as defined in the appended claims.

Iclaim:

1. An artificial transmission line comprising two elongated layers ofelectrically conducting materials separated by and convolutely woundwith dielectric spacing material, one of said layers of electricallyconducting material being directly connected on both sides throughoutsubstantially its entire length to a first electrically conductiveterminal and the other of said layers of electrically conductingmaterial being directly connected at its inner extremity only to asecond electrically conductive terminal which extends from both sides ofthe winding.

2. An artificial transmission line comprising a wide electrode foil anda narrow electrode foil separated by and convolutely wound with layersof dielectric material of intermediate length, the wide electrode foilbeing directly connected on both sides throughout substantially itsentire length to a first electrically conductive terminal and the narrowelectrode foil being directly connected at its inner extremity only to asecond electrically conductive terminal which extends axially from bothsides of the winding.

3. An artificial transmission line comprising a wide electrode foil anda narrow electrode foil separated by layers of dielectric material andconvolutely wound upon an electrically conducting core which extendsbeyond both sides of the winding, the narrow electrode foil beingdirectly connected at its inner extremity to said core and the wideelectrode foil being connected on both sides throughout substantiallyits entire length to a metallic housing for said winding.

4. An artificial transmission line comprising a wide elongated electrodefoil and a narrow elongated electrode foil separated by layers ofdielectric material of intermediate width, the longitudinal centers ofeach of said foils being in register and said foils being convolutelywound on an elongated metal core which extends beyond both edges of saidwinding, the inner turn of said narrow foil electrically engaging saidcore and the projecting edges of the wide foil being embedded in solderconnected to a cylindrical metal housing which encloses said convolutelywound assembly.

5. An electrical circuit useful as a transmission line and in bypassingundesired electric signals of high frequencies, said circuit comprisingt .e artificial line of claim 4 wherein said housing and one end of saidcore are connected as input terminals and the metal housing and theother end of said core are connected as output terminals.

NELSON E. BEVERLY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,259,234 Voight Oct. 14, 19412,440,652 Beverly Apr. 23, 1948 2,466,766 Hartzell Apr. 12, 1949

